Uplink (UL) interference can cause problems for a base station that is attempting to receive and process a transmitted uplink signal from a user equipment in a wireless communication system. Throughout this document the term “station”, such as used in base station, macro station, macro base station, femto base station, and femto station, is intended to denote the communications equipment. The term “cell”, such as used in femtocell or macrocell, is intended to denote the coverage footprint of a corresponding station or the coverage footprint of a sector of a multi-sector base station.
Broadband wireless cells may be UL interference limited. There are many scenarios that can cause UL interference. The most common scenario has historically been cell edge interference in a frequency “reuse one” network or when neighboring cells share a frequency channel, causing co-channel interference. This interference scenario is well known in the art.
However, the Long Term Evolution (LTE) wireless communication standard adds an additional very important scenario. When a femto base station is present in the macrocell coverage footprint of a macro base station in a frequency “reuse one” network, this can create what is termed in LTE as “the near-far problem.” If a user equipment (UE) is in the femtocell coverage area of the femto base station, but is in communication with the more distant macro base station, it may be transmitting using a very high power thereby causing excessive uplink interference at the femto base station. Other uplink interference scenarios exist as would be known to one skilled in the art.
Aside from brute force methods, such as using very robust and inefficient modulation and coding schemes, current methods to combat this problem include coordinated multipoint (CoMP) which contains multiple methods. First, fractional frequency reuse may be used, coordinating the UL resources so that a UE communication with a macrocell and a UE communication with a femto base station that has an overlapping coverage footprint do not transmit on the same subcarriers simultaneously. This technique has been previously used in WiMAX. Second, the two base stations may use beamforming to coordinate the UL resources spatially. Both of these methods require coordinated scheduling. Additionally, using joint reception, both base stations may receive the same data from an individual UE using the same subcarriers at the same time. This method may additionally use beamforming. The joint reception, of course, uses double the resources because the resources of both base stations are tied up with the same reception.
Fractional frequency reuse is inefficient due to the need for one base station to not schedule UL resources while the other is using them. Beamforming requires significant antenna resources which may not be available on a femto base station. Additionally, there may be times when coordinated scheduling of beamforming cannot be achieved due to the bandwidth and QoS/QoE needs of interfering UEs, requiring a fallback to fractional frequency reuse.
There exists a need for a spectrally efficient (e.g., efficient modulation and coding, transmitting using all available time and frequency resources) method to resolve UL interference as an alternative to existing spectrally inefficient solutions which merely mitigate UL interference.